The present invention relates generally to mobile material handling machines that are used to lift and place material, and more particularly, to such a machine having a device for stabilizing the machine during manipulation of its material handling implement.
Mobile mounted material handling machines such as cranes, forklifts, and excavators have been manufactured with and without suspension systems. Machines with suspension systems are more suitable for driving to the worksite because they permit at least one axle or tire set to oscillate relative to the vehicle frame when driving over uneven terrain. While this suspension improves the quality of ride and reduces frame stresses, it also limits the lateral stability of the machine when lifting material transverse to its longitudinal axis. Machines without suspension systems are more suitable to resist the overturning forces when lifting material transverse to their longitudinal axis because the outboard end of the axle can resist the overturning loads on the machine.
To overcome this inherent stability deficiency of machines with suspension systems, manufacturers have incorporated vehicle stabilizers that can be positioned external to the wheelbase of the machine when the machine is handling material. More recently, manufacturers have utilized hydraulic cylinders to control and limit the movement of suspension systems, thereby increasing the transverse stability of the machine, when it is used for handling material.
In U.S. Pat. No. 4,174,854, a vehicle stabilizing apparatus is described and includes both "spring" and "locking" cylinders, levelling valves and other related hardware to select between a rigid and "floating" suspension system. In U.S. Pat. No. 4,705,295, a vehicle stabilizing system is disclosed that requires air to be present to lock the suspension system in its most stable condition. This method of locking the suspension system is potentially dangerous because loss of air pressure due to a leak or broken air line could cause the vehicle to lose its "locked" condition and cause the vehicle to overturn and cause personal injury and/or property damage.
It is desired to eliminate this inherently dangerous condition by using a valving arrangement that locks the axle and machine in its most stable configuration without the requirement of air pressure in the pneumatic lines. It is further desired to provide a hydraulically actuated locking suspension system.